Cubone's skull looks loose-fitting, like it can fall off any second. Its eye socket doesn't fit, and it shows skin.

When it evolves into Marowak, its skull gains a sleeker shape. It looks tightly fitted, almost impossible to take off. Its eye socket aligns perfectly with the eye, almost as if the eye is part of the skull. If you look at Marowak from the side, like in this picture, you can see that the skull has fully enveloped the head.

When Cubone evolves into Marowak, its skull adapts to fit, and basically becomes part of its body.

ITT: Cool shit you've noticed about Pokemon.